Exercising in the Heat and Sun

Abstract

Because of the many health benefits of physical activity, it is recommended that Americans engage in moderate-intensity

exercise on a regular basis.35 However, exercising in a hot environment places an increased strain on the body and increases

the risk for heat-related illness or injury. Understanding how the body responds and adapts to being physically active in the

heat is critical to ensuring safe and effective exercise. The purpose of this review is to discuss (1) how exercising in the heat

and sun can be a major challenge to body temperature regulation, (2) physiological and behavioral adaptations to combat the

threat of the potential illness or injury related to exercising in a hot environment, and (3) populations and predisposing factors

that may be associated with an increased risk for heat-related problems. While the term “exercise” is used throughout this

review, it is important to note that the information provided is relevant for anyone who is physically active in the heat,

whether exposure to a hot environment occurs during sports competition, recreational exercise, work, or summer vacation.

Introduction: Challenges to Exercising in the Heat and Sun

Body Core Temperature and Hyperthermia

To preserve optimal physiological function, human body core temperature is usually regulated within a relatively narrow

range (35-39°C or 95-102.2°F).2 During resting thermoneutral (mild environmental temperature) conditions, body core

temperature (set point = 37°C or 98.6°F) is maintained by an equal rate of body heat gain and heat loss. However, when the

rate of heat gain is greater than the rate of heat loss, body core temperature will increase above set point, and in extreme

situations, rise to dangerously high levels (> 40°C or 104°F). The primary means

by which the body gains heat is from the environment and metabolism. When

ambient temperature is greater than skin temperature (~33°C or 91.4°F at rest, up

to ~36°C or 96.8°F during exercise) heat will be transferred from the surrounding

air to the body. When exercise is performed, a large amount of heat is produced

by the contracting muscles. In fact, less than 25% of all the energy produced by

contracting muscles is used to perform work, with the remaining 75% converted

to heat in the muscles. The combination of exercise and high environmental

temperature (hereafter referred to as exercise-heat stress) presents a serious

challenge to body core temperature regulation.17, 39

The means by which the body gains and loses heat during exercise are

summarized in Figure 1. Metabolic heat production is directly proportional to

exercise intensity. Radiation is the transfer of heat between two objects, with no

physical contact being involved. The direction of radiant heat transfer depends on

a thermal gradient (e.g., body heat loss from the skin to the environment occurs

when skin temperature is greater than air temperature and heat is gained by the

body when solar energy from direct sunlight is absorbed by skin). Convection is

the heat exchange between the body and surrounding moving air (wind) or body

fluids (blood). In evaporation, heat is transferred from the body to water (sweat)

on the surface of the skin. When this water gains sufficient heat, it is converted to

a gas (water vapor), thereby removing heat from the body. Evaporation of 1 kg of

sweat from the skin will remove 580 kcal of heat from the body.44

An increase in body temperature is sensed by central and skin thermoreceptors

and this information is processed by the hypothalamus to trigger appropriate

responses. The two basic physiological mechanisms by which humans dissipate

excess heat are by (1) increasing skin blood flow, thereby allowing heat to be

moved via convection from the body core to the skin and (2) secreting sweat onto 

Series 8, No. 2 June 2007

Published quarterly by the

President’s Council on

Physical Fitness and Sports

Washington, D.C.

★

Guest Authors:

Lindsay B. Baker and

W. Larry Kenney, Ph.D.

Noll Laboratory, Department of

Kinesiology, The Pennsylvania State

University, University Park, PA

★

Co-edited by:

Dr. Barbara Ainsworth,

Arizona State University,

Dr. Deborah R. Young,

University of Maryland, and

Dr. Michael La Monte,

University of Buffalo,

The State University

of New York

President’s Council on Physical Fitness and Sports

ResearchDi est
Figure 1.

Schematic diagram showing the means by which

the body gains and loses heat during exercise16

the surface of the skin for subsequent evaporation. Heat

loss via these two mechanisms increases in proportion to

the rate of heat production and usually increases

sufficiently to balance metabolic heat production,

allowing a new steady-state (relatively constant) body

core temperature to be achieved (slightly higher than

resting body core temperature, e.g., ~38°C or 100.4°F).

Of these two heat loss mechanisms, the most important

during exercise is evaporation of sweat. In fact,

when exercise is performed in an air temperature

greater than skin temperature, evaporation is the

only means of losing body heat. A key

determinant of evaporative cooling capacity is the

water vapor pressure gradient between the skin

and the air. When the skin surface is saturated

with sweat it has a high water vapor pressure (i.e.,

100% relative humidity); thus, if ambient

humidity is low (e.g., 20% relative humidity) the

water vapor pressure gradient between the skin

and the air is large and would permit a relatively

high rate of evaporative cooling. By contrast, high

ambient humidity (e.g., 80% relative humidity)

results in a smaller vapor pressure gradient

between the skin and the air, thereby reducing the

rate of evaporation of sweat and severely limiting

the body’s cooling capacity. It should be noted

that sweat dripping from the body is wasted water

loss because sweat must evaporate to allow

effective cooling. During vigorous exercise in an

environment of high heat and humidity, body core

temperature will not reach a steady state, but

continue to rise, making the continuation of

exercise especially difficult and potentially

dangerous.17, 39

Cardiovascular Responses

The cardiovascular system is also important in

temperature regulation. The demand for increased

blood flow to both the exercising muscles and skin

during exercise-heat stress places an increased strain on

this system. For instance, the increase in skin blood flow

during exercise-heat stress is accomplished by (1) an

increase in cardiac output (volume (e.g., mL) of blood

ejected from the heart each min) above that of exercise in

cool conditions and (2) the redistribution of blood away

from the gastrointestinal tract, hepatic, and renal

circulations toward the skin circulation. Additionally,

sweat loss during exercise-heat stress can cause a decrease

in blood plasma volume. A smaller overall plasma volume

will result in a reduction in the amount of blood that is

returned to the heart (cardiac filling or venous return) and

thus a reduction in the amount of blood that is ejected

from the heart with each beat (stroke volume). A

decreased stroke volume requires a compensatory increase

in heart rate to maintain cardiac output. In this manner, a

decrease in plasma volume causes cardiac strain and

limits the amount of blood flow available to the muscles

and skin during prolonged exercise-heat stress.37, 39

Heat-Related Problems 

Table 1 describes the possible problems that may arise as

a consequence of exercising in a hot environment.2, 8 Heat

exhaustion, syncope, and cramps are relatively minor and

common problems that can usually be resolved by

cessation of exercise, replacing fluid and electrolytes, and

relocating to a cooler environment. These problems are

2

Table 1.

Injuries and illnesses associated with exercising in the heat.

Condition Symptoms Causes

Muscle (Heat) Cramps acute, painful, sodium deficiency,

involuntary muscle neuromuscular

contraction fatigue, dehydration

Heat Exhaustion inability to continue dehydration,

exercise, fatigue, hyperthermia,

weakness, sodium deficiency,

hyperventilation, energy depletion

headache, nausea,

diarrhea

Heat Syncope fainting after peripheral pooling

sudden cessation of blood,

of exercise or dehydration,

sudden/prolonged cerebral ischemia

standing, headache,

nausea

Heat Stroke central nervous thermoregulatory

system overload or

dysfunction failure

manifested as

altered mental

status

(e.g., confusion,

combativeness),

seizures, coma
When individuals underdrink during exercise-heat

exposure, they become hypovolemic (overall decrease in

blood plasma volume) and hyperosmotic (an increase in

the concentration of solutes in plasma, because of

increased water lost as sweat). On the other hand, if

someone overdrinks (drinks more fluid than they have lost

through sweating), he or she can become hypervolemic

(overall increased volume of plasma) and hypo-osmotic

(dilution of plasma). The gross overconsumption of fluids

(especially sodium-free fluid, such as water) over a

prolonged period of exercise (usually > 4hrs) can lead to

the dilution of sodium concentration in the blood. This

rare, but potentially dangerous condition is called

hyponatremia, and is defined as plasma sodium

concentration less than 135 mmol/L. Normal values are

135 to 145 mmol/L. The reduction in solute concentration

in plasma promotes movement of water from the plasma

and into cells, which can cause swelling in the brain,

alters central nervous system function, and congestion in

the lungs.31 Symptoms of mild to moderate hyponatremia

may include headache, nausea, dizziness, and muscle

weakness, while severe hyponatremia (typically plasma

sodium concentration < 125 mmol/L) is characterized by

pulmonary edema, cardiorespiratory arrest, cerebral

edema, seizures, and/or coma.5, 31

Exercise Performance

It is well established that heat stress and/or dehydration

result in an earlier onset of fatigue during prolonged

aerobic exercise compared to exercise in cool conditions

and/or in a euhydrated state.19, 38 There is also evidence

that performance of intermittent, high-intensity exercise

tasks, including skill sports (e.g., soccer and basketball), is

negatively impacted by heat stress and/or dehydration.13, 29, 32

Dehydration exacerbates hyperthermia by impairing

evaporative and convective cooling capacity and

increasing circulatory strain.38 Additionally, blood flow to

exercising muscles is attenuated with dehydration during

exercise-heat stress.20 Hyperthermia causes a shift in

metabolism; such that muscle glycogen oxidation and

blood lactate accumulation occur at a faster rate than if

exercise was performed in a cool environment.27, 39 Any of

these factors could contribute to the early onset of fatigue

during exercise-heat stress; however, the most important

may be central nervous system changes that occur as a

result of hyperthermia. According to the critical internal

temperature hypothesis, individuals exercising in the heat

consistently reach the point of exhaustion at the same

threshold body core temperature (~40°C or 104°F). This

critical core temperature coincides with voluntary fatigue

despite variations in exercise intensity, initial core

temperature, adiposity, acclimation, or hydration status.

The concept underlying the critical core temperature

hypothesis is that an increased body core and brain

temperature is associated with a decreased central drive to

exercise.

33 The diminished drive to exercise is associated

with an increased rating of perceived exertion and reduced

motor unit recruitment and firing rate to the exercising

muscle. From an evolutionary perspective, central fatigue

associated with normal to slightly elevated body core

temperature. On the other hand, heat stroke is a

potentially life-threatening condition in which body core

temperature rises to > 40°C (104°F) and requires

emergency medical attention and rapid body cooling. All

of these heat-related problems are preventable with heat

acclimation, proper hydration practices, and behavioral

adaptations (prevention is discussed in more detail later).

Fluid and Electrolyte Imbalances

When fluid intake is less than sweat loss a body water

deficit, or dehydration, occurs. It is difficult to closely

match the volume of fluid intake to the volume of sweat

output because thirst does not provide a good index of

body water requirements. Ad libitum drinking (i.e.,

drinking in response to thirst stimulus) during and/or after

exercise-heat stress usually results in incomplete fluid

replacement, a concept known as “voluntary

dehydration”. In fact, it is common for individuals to

voluntarily replace only about 50% of sweat losses during

a given bout of exercise in a hot environment.8, 11

Water accounts for approximately 60% of total body

weight in the average adult. A reduction in body water is

associated with impaired circulatory and thermoregulatory

function during exercise. For instance, dehydration causes

a decrease in plasma volume and therefore a decrease in

stroke volume and a compensatory increase in heart rate

to maintain a given cardiac output at a given work rate.38

Body water loss also leads to a decreased capacity to

sweat and augment skin blood flow.15 By impairing heatdissipating mechanisms and circulatory performance,

dehydration results in an increased core temperature. The

magnitude of hyperthermia depends on ambient

conditions and exercise intensity, but the range is 0.1 to

0.4°C (0.2 to 0.7°F) per 1% dehydration. Thus, the higher

the percent dehydration at a given exercise intensity and

ambient temperature, the higher body core temperature

will rise and the higher the risk for heat-related injury and

illness. For a 70 kg (154 lb) individual exercising at a

moderate intensity (55-70% maximal heart rate) on a hot

day (85°F and 50% relative humidity), sweating at a rate

of 1.0-1.5 L (2.2-3.3 lb) per hour, it would take about 60-

80 min (without fluid replacement) to reach 2%

dehydration, or about 120-160 min (without fluid

replacement) to reach 4% dehydration. If sweat losses are

fully replaced, body core temperature will typically

stabilize at about 38.0°C (100.4°F). Conversely, if the

individual becomes 2-4% dehydrated, body core

temperature will continue to rise to > 38.5-39.0°C

(>101.3-102.2°F). Body core temperature may reach

higher levels (>40°C or 104°F) if exercise intensity, air

temperature, and/or relative humidity is increased (above

that of the conditions in the example provided). As little

as 2% dehydration is associated with mental/physical

fatigue and impaired exercise performance.4, 10, 13, 21 Other

signs and symptoms of dehydration are headache, mild

nausea, or dizziness.8

3
4

acts as a safety mechanism—by decreasing the drive to

exercise and therefore decreasing the rate of muscle

contraction—hyperthermia is minimized and dangerous

increases in core temperature are prevented.33

Combating Hyperthermia 

Heat Acclimation

One of the best methods for improving heat tolerance and

decreasing risk of developing a heat illness or injury is to

gradually acclimate to exercise in hot environments. Heat

acclimation is a series of physiological adaptations that

occur with repeated exposure to exercise-heat stress that

makes humans better able to prevent dangerous increases

in core temperature. Table 2 summarizes the adaptations

that occur as a result of heat acclimation relative to a nonacclimated state.43 Improved heat tolerance will occur

within 1-2 weeks of aerobic exercise in a hot environment.

When beginning a heat-acclimation regimen it is

important to start slow (walking or jogging for as little as

10-15 min) and gradually increase the intensity and

duration (up to 90 minutes) of exercise in the heat.2

Table 2.

Summary of adaptations that occur after 1-2 weeks

of repeated exercise-heat exposure.

Sweating

Earlier onset

Increased sweating rate

More uniform sweating pattern over entire body

surface area

Decreased sodium concentration

Circulation

Plasma volume expansion

Stroke volume increases

Heart rate decreases

Decreased core and skin temperature

Improved exercise performance or work capacity

Decreased rating of perceived exertion

Decreased mental fatigue

An earlier onset of sweating and increase in sweating rate

enhances the body’s cooling capacity, while a reduced

sweat sodium concentration minimizes the risk for heatrelated injury and illness associated with sodium

deficiency. Prior to heat acclimation, local sweating rate is

higher on the torso than the limbs. Repeated exercise-heat

exposure results in a more uniform sweating pattern over

the entire body surface area (by increasing sweating rate

of arms and legs). Higher evaporation of sweat over the

limbs may be more advantageous than that over the torso

because of a larger surface area-to-mass ratio on the

limbs. Plasma volume expansion occurs as a result of

plasma sodium retention (via sweat sodium conservation)

and translocation of protein and fluid from the interstitial

space to the plasma. An increase in plasma volume

reduces cardiovascular strain during exercise-heat stress

by increasing stroke volume and decreasing heart rate

compared to that of a non-acclimated state.43

All of the adaptations in sweating and circulatory function

attenuate the increase in body core temperature during a

given exercise-heat exposure, thereby improving

physiological and psychological tolerance of exercising in

the heat and lowering risk of heat-related illness or injury.

Passive heat exposure or exercise training without heat

exposure offers some but not all of the benefits of

repeated exercise and heat stress combined. It is important

to note that adequate fluid and sodium intake is necessary

to experience complete heat acclimation (fluid and

sodium replacement discussed in more detail later). Once

heat exposure stops, the benefits of acclimation are

retained for approximately 1-4 weeks. Sleep loss, alcohol

abuse, salt depletion, and dehydration result in a rapid

decay of acclimation, while high levels of aerobic fitness

are associated with greater retention of heat acclimation.34, 43

Behavioral Adaptations

The risk of developing heat-related problems can also be

reduced by properly adjusting the exercise program to

account for the effects of the environmental conditions.

When making exercise decisions based on the thermal

environment, it is important to evaluate all aspects of the

environment which impact exercise safety. Ambient

temperature, humidity, air movement, and solar radiation

are all important factors to consider. The outdoor Wet

Bulb Globe Temperature (WBGT) is a useful index of

environmental heat load as it represents a composite

measure of all four factors. The WBGT can be calculated

as follows:3

WBGT = 0.7 Twb + 0.2 Tbg + 0.1 Tdb [°C or °F]

(Equation 1)

Where Twb is measured by placing a wetted cotton wick

over the thermometer bulb (measures combined effect of

humidity and air velocity), Tbg is the temperature inside a

copper globe painted flat black (measures radiant heat

load), and Tdb is the direct measure of air temperature. Twb

makes the highest contribution to this heat stress index

because, if the ambient water vapor pressure (i.e.,

humidity) is high and wind velocity is low, then the

evaporation of sweat from the skin surface is limited.

Directors of outdoor activities (e.g., road races) are

encouraged to use the WBGT heat index chart to make

decisions about whether or not to hold their events. The

four categories of risk for heat illness or injury include:3

Very high risk: WBGT above 28°C (82°F);

High risk: WBGT 23-28°C (73-82°F);

Moderate risk: WBGT 18-23°C (65-73°F);

Low risk: WBGT below 18°C (65°F).

It is important to note that these measures of risk are

based on individuals wearing shorts and a T-shirt. The

wearing of additional clothing can greatly impede heat
dissipation and increase heat stress. If available, lightcolored (to minimize solar radiant heat gain), loose-fitting

(for optimum ventilation, as tight or poorly ventilated

clothing reduces the convective flow of air over the skin

surface) clothing made of moisture-wicking fabric (to

allow evaporation of sweat) should be worn while

exercising in the heat.18 To prevent sunburn, individuals

should minimize exposure to the sun from 10 am-2 pm,

wear a hat, and generously apply a sunscreen with SPF 

> 15 about 30 minutes before sun exposure.2, 14, 40

It should also be noted that the risk for heat-related

problems is much higher if exercise is vigorous (e.g.,

> 70% of maximal heart rate), since metabolic heat

production drives the increase in core temperature. In fact,

exertional heat stroke often occurs in overzealous

individuals who ignore safety recommendations and fail

to make appropriate behavioral adjustments while

exercising in hot and/or humid conditions. On especially

hot and/or humid days exercise intensity should be

reduced. This can be accomplished by slowing the pace or

adding rest breaks to the exercise bout.8 Another

technique for maintaining a safe level of exercise intensity

involves the use of target heart rate. As discussed

previously, heart rate is higher during exercise in the heat,

compared to exercise in cool conditions. In fact, heart rate

increases by 5 beats per minute for every 3-4°C (5.4-

7.2°F) increase in WBGT.17 Adjusting exercise pace to

stay within the target heart rate zone will cause an

appropriate corresponding decrease in exercise intensity.

Fluid and Electrolyte Replacement

To prevent the circulatory and thermoregulatory strain and

impaired exercise performance associated with

dehydration, the recommended hydration strategy is to

drink enough fluid to fully replace sweat and urine losses

incurred during exercise.11 A simple method to estimate

fluid loss is to measure body weight before and after a

workout. The goal during exercise and recovery should be

to drink just enough fluid to maintain pre-exercise body

weight (i.e., drink 16 oz of fluid for every lb of sweat

lost). Sweating rates can vary from less than 0.5 to over

2.5 L (17 to over 85 oz) per hour depending upon exercise

intensity, environmental conditions, amount and type of

clothing or equipment, acclimation state, fitness level, and

hydration status.38 Because of this considerable variation,

it is important to know the individual’s sweating rate to

avoid the negative effects of drinking too little or too

much fluid.

The composition of fluid consumed during and after

exercise-heat stress is also an important consideration.

Prolonged and/or intense (especially stop-and-go type

sports, such as soccer, basketball, or tennis) exercise can

cause substantial fluid and electrolyte loss and reliance on

carbohydrates as the primary fuel for muscles. Thus, a

fluid replacement beverage with carbohydrates and

electrolytes should be consumed for optimal performance

during these types of activities.28, 41 Carbohydrates supply

energy to contracting muscles and also provide flavor

which enhances palatability and promotes voluntary fluid

consumption. Solutions with a carbohydrate concentration

< 6% are emptied from the stomach and absorbed from

the intestine into the bloodstream at rates similar to that of

plain water. Sodium ingestion during and after exercise

helps maintain thirst (resulting in greater voluntary fluid

consumption), protects plasma volume, maintains plasma

sodium concentration, and reduces urine production.11, 23

These responses aid in keeping exercisers well-hydrated,

help sustain cardiovascular function,12 and reduce the risk

of electrolyte imbalances (e.g., hyponatremia).30 It is

important to note that exercisers who consume sports

drinks should still take care to drink only enough fluid to

maintain initial (i.e., pre-exercise) body weight. Although

sports drinks contribute sodium to the blood, the sodium

concentration of sports drinks is lower than that of the

blood, so drinking substantially more fluid than is lost in

sweat could eventually result in the dilution of blood

sodium concentration (i.e., hyponatremia).

Considerations for Special Populations

Older Adults

Epidemiological data suggest that advanced age (>65 yrs)

is associated with an increased risk of heat-related death.9

Because of age-related changes in skin structure, older

individuals have an attenuated sweat gland output and

skin blood flow response to heat stress. However, studies

which match healthy older and younger persons for body

composition, aerobic capacity, and state of heat

acclimation have concluded that older individuals respond

to heat stress with a rate of heat storage and final core

temperature similar to their younger counterparts.24

Likewise, older athletes and young adults are equally

capable of acclimating to exercise in a hot environment.36

These findings suggest that it is not chronologic age per

se that causes impaired temperature regulation, rather it is

other factors which change concomitantly with advancing

age that put older men and women at an increased risk for

heat-related illness or injury. Examples of variables that

typically change with age and that could affect heat

tolerance include (1) decreased aerobic capacity,

(2) sedentary lifestyle, (3) increased prevalence of chronic

diseases (e.g., diabetes and cardiovascular disease), and

(4) increased use of prescription drugs (e.g., diuretics and

anticholinergics). Nonetheless, for healthy older men and

women who maintain a high degree of aerobic fitness, the

risk of heat-related illness is not significantly greater than

that of young adults.24 It is important to note, however,

that since aging is associated with reduced thirst

sensitivity in response to dehydrating exercise in a warm

environment and increased water excretion by the kidney,

adequate fluid intake should be particularly emphasized

for older individuals exercising in the heat and sun.26

Children

In general, children are thought to tolerate exercise in the

heat more poorly than adults. Several physiological

5
illness due to dehydration or fever. Additionally,

individuals with a prior episode of heat stroke may be

more susceptible for recurrent heat illness.2, 8

Sleep Loss—Prolonged sleep loss has been shown to

decrease skin blood flow and sweating rate at a given core

temperature.2

Chronic Disease—Medical conditions that can raise the

risk of incurring heat-related problems include

hypertension (alters the control of skin blood flow),

diabetes (neuropathies may affect sweating and/or skin

blood flow), obesity (increased rate of metabolic heat

production during exercise), and cystic fibrosis (increased

salt loss in sweat).2, 8

Medications and Drugs—Certain medications and drugs

may predispose an individual to heat-related illness by

promoting heat storage or impairing heat loss

mechanisms. For example, antihistamines and

anticholinergics reduce sweat production, beta-adrenergic

and calcium-channel blockers impair skin blood flow,

amphetamines and tricyclic antidepressants increase

metabolic heat production, and alcohol, caffeine, and

theophylline (found in tea) are mild diuretics (stimulate

urinary fluid and sodium loss).2, 8, 42

Sunburn—When exercising in a hot environment,

sunburned individuals exhibit an impaired ability to

secrete sweat. In addition, sunburn increases ratings of

perceived exertion and thermal sensation, which means

that sunburned individuals feel more hot and fatigued

when exposed to a given exercise-heat stress.2

Summary

Body core temperature during exercise-heat stress is

determined primarily by the balance of environmental

conditions (e.g., temperature, humidity, air velocity, and

radiant heat load) and the body’s heat dissipating

mechanisms (e.g., skin blood flow and sweating). If body

heat gain exceeds heat loss and/or if fluid intake is not

sufficient to match sweat loss, problems related to

hyperthermia and/or dehydration (e.g., muscle cramping,

heat exhaustion, heat syncope, and heat stroke) can

develop. Predisposing factors such as age (older adults or

children), disease, and medication/drug use, may put

certain individuals at an increased risk for heat illness and

injury. However, physiological (e.g., heat acclimation) and

behavioral adaptations (e.g., avoiding exercise when

WBGT > 73°F, wearing light-colored, loose-fitting,

moisture-wicking clothing, decreasing exercise intensity,

consuming 16 oz of fluid for every lb of sweat lost) can

minimize the risk for heat-related problems and thus

facilitate the safe participation in regular exercise.

6

characteristics put children at a thermoregulatory

disadvantage. First, sweating capacity (sweating rate per

unit body surface area and sweat production per gland) is

much lower in children than adults. In addition, children

experience greater cardiovascular strain during exerciseheat stress, as cardiac output per unit of oxygen uptake is

lower and heart rate is higher in children than adults.

Furthermore, compared to adults, children acclimate to

the heat at a much slower rate and experience a greater

increase in body core temperature when hypohydrated.

For reasons not entirely clear, children expend more

energy (and thus produce more metabolic heat) per kg

body mass than adults at any given walking or running

speed. Children also have a higher body surface area-tomass ratio than adults; and since heat transfer between the

environment and the body depends on the surface area of

the body exposed to the environment, children absorb heat

faster than adults whenever ambient temperature exceeds

skin temperature. Having a higher body surface area and

lower sweating capacity means that children rely to a

greater extent on radiation and convection rather than

evaporation of sweat to dissipate body heat. As long as the

ambient temperature is no more than 5 to 7°C (9 to

12.6°F) above skin temperature (i.e., ambient temperature

<42°C or 107.6°F), children regulate their core

temperature as effectively as adults during < 1 hr of

exercise. However, in extremely hot environments

(> 45°C or 113°F) children’s ability to dissipate heat is

compromised and exercise tolerance is significantly lower

than that of young adults.7

Other Factors

Sex—In women and men who are matched for fitness,

acclimation state, adiposity, and body size, there does not

appear to be sex-related differences in thermoregulatory

function other than a lower sweating rate in women as

compared to men.25 While having a lower sweating rate is

a thermoregulatory disadvantage in hot-dry environments,

it is actually an advantage for women in hot-humid

conditions because they have a lower risk of dehydration.

The menstrual cycle has minimal effects on tolerance to

exercise-heat stress.25

Hyponatremia occurs more commonly in women;

however, evidence suggests that this is most likely due to

sex differences in body size, racing time, and fluid intake

behavior, rather than sex per se. Women tend to have

smaller body mass (and less total body water), slower

marathon times (>4 hrs, which means they have more

time to drink), and voluntarily drink more fluid per body

mass than men, which puts women at an increased risk for

developing hyponatremia.1, 6, 22

Recent Illness—Individuals who are currently or were

recently ill may be at an increased risk for heat-related
7

Please Post

Lindsay B. Baker and W. Larry Kenney, PhD

Noll Laboratory, Department of Kinesiology

The Pennsylvania State University

University Park, PA

Exercising in a hot environment places

an increased strain on the thermoregulatory

and cardiovascular systems and increases

one’s risk for heat-related illness or injury.

Understanding how the body responds

and adapts to being physically active

in the heat is critical to ensuring safe

and effective exercise.
8

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25. Kenney, W.L. A review of comparative responses of men and women to

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26. Kenney, W.L. and P. Chiu. Influence of age on thirst and fluid intake.

Medicine and Science in Sports and Exercise 33(9):1524-1532, 2001.

27. MacDougall, J.D., W.G. Reddan, C.R. Layton, and J.A. Dempsey. Effects

of metabolic hyperthermia on performance during heavy prolonged

exercise. Journal of Applied Physiology. 36(5):538-544, 1974.

28. Maughan, R. Carbohydrate-electrolyte solutions during prolonged

exercise. In: Perspectives in Exercise Science and Sports Medicine:

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Lamb and M.H. Williams (Eds.) Indianapolis, IN: Benchmark Press,

1991, pp. 35-85.

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